Saturday, 6 February 2010

Science Report

Object: To discover whether heavy boats 'carry their way' or just slow you down.
Method: A boat was rowed over a course in Portsmouth Harbour, starting at Hardway and rounding a massive buoy that usually has a dead frigate attached to it. The first run was with no ballast, then with two bags of sand, then with four bags. Speed and course were recorded on a Nokia E71 mobile phone using Sports Tracker software.
Results:
First Lap (No ballast): Distance 0.71km, Ave speed 6.98km/h, Top speed 10.9km/h
Second Lap (Two bags of sand): Distance 0.72km, Ave speed 6.60km/hr, Top speed 11.5km/hr
Third Lap (Four bags of sand): No results due to operator error (I think I pressed the wrong button. Sorry, Sir, honest it won't happen again).
All the tracks can be seen here.
Impressions: The ballast definitely held me back when getting going, and seemed to be harder work. Also, turning seemed to be more difficult. But the extra weight really did seem to carry the boat forward during the return.
Conclusion: Adding ballast clearly slowed the boat up, but not by much. The boat sat significantly lower in the water, which was slightly alarming. All in all, the conclusion is that the extra momentum of the ballast is more than compensated for by the extra work propelling it, but more research is necessary (SRI grant please!).
After the science stuff, Martin Corrick took me round Portsmouth Harbour in his spiffy Topper Cruz:

And just to warm up after an hour sitting still sailing, I rowed all the way round the former HMS Southampton, sitting on her buoys awaiting her ultimate end.

8 comments:

John Hesp
said...

This is interesting and something I've thought a lot about, but haven't come to any conclusions about.

On the face of it the heavier boat needs more force/energy to propel it.But I think this fact is slightly blurred by the fact that a rowing boat doesn't travel at a constant speed; that is, the boat accelerates during the power stroke, and decelerates during recovery.This is a bit wasteful as the energy needed to go faster than the average speed is more than the energy saved when going slower than the ave speed. Better to travel at the ave speed all the time, which adding weight tends to do.

However much inconclusive pondering I do on this there's no escaping the fact that most competion rowing craft go to extroadianary lengths to keep weight to a minimum, which is probably proof enough.

One of the exceptions to this is the Cornish Rowing Gig. When asked about building a gig using modern lightweight materials the late Ralph Bird replied that a lighter boat wouldn't carry its way, and would be slower.Assuming Ralph was right (and he was probably repeating what trial and error over centuries had revealed) what's the difference I wonder?I can only think that it might be beneficial to have a slightly heavier boat in choppy seas. A Boat moving into a chop with a consitant power source will experience acceleration/deceleration. A rowing boat is going to experience acc/dec from the chop, and acc/dec from the oars. What if these two effects work together to bring you to a standstill?

Anybody rowing a small rowing boat into a large chop will know that you have to time your strokes very carefully. I always found that I needed to pull up the face of the oncoming wave - accelerating the boat as the wave decelerated the boat. I don't think this would be possible on a six oared boat. Maybe in this particular case it pays to have a bit of additional weight to maintain a more consistant speed.

Also, these boats travelled some distance. It takes less energy to travel (say) 7 miles at 7 knots than starting at 10 knots and gradually slowing down to 4 knots. Again, the above ave speed uses more extra energy than the below ave speed saves energy, and the heavier the boat the truer this is.The hundred yard sprinter runs as fast as he can. The marathon runner paces himself.So if a bit of extra weight was found of some use in rowing upwind in a chop, it wasn't such a drawback to a boat which travelled many miles at a reasonable speed as it would be to a boat travelling a short distance at max speed.

All in my opinion of course. I'd like to hear what others think. I think you'll have to repeat the experiment in a chop Chris.

I seem to recall F=MA where F is force (Chris's effort), M= mass (weight to the rest of us) and A is acceleration.

Now boats on water are a low friction environment, but sooner or later the hull friction will slow you to a stop. It therefore results that if you lower mass, more of Chris's energy is converted into acceleration i.e. overcoming friction.

That's fine for flat water, but in any kind of chop, A (acceleration opposing motion) will increase dramatically when a wave strikes, in that case a higher Mass would (might) reduce the force of the wave as long as Chris's effort remains constant.

That's the theory ( as I remember it) but of course Chris might just have beena little more tired on his second circuit.

Don't forget waterline. Depending on the hull shape, the waterline increases as the boat rides lower (but more horsepower is required to push it).

Years of rowing have convinced me that windage has a lot to do with forward momentum, perhaps more than weight. A lightweight rowing dory will take more effort to maintain the same speed as a kayak in a headwind.

Anther thing a rower must contend with is the (reverse) inertia of the return stroke. So many factors!!

I've been following this discussion on the HBBR group, and there are as many theories as theorists. Perhaps three rowers of similar skill, with similar boats and different weights should row the course simultaneously. A race, if you will. Best of three, trade off boats each time.

Purely from my experience with the Light Trow I would advocate the lighter boat even in waves. I’ve rowed fairly long distances with just the bare hull and also with the masts stepped and the boat loaded for cruising.

As you’d expect, the bare hull in flat water with no wind is faster. In waves (and wind) it is slowed but is easy to accelerate in the troughs and lulls. The heavier boat is also slowed, though maybe not as much, but requires far more effort to get going again. Over any significant distance I will tire faster with the heavier boat and row more slowly as a consequence.

Thanks for all the input, guys. I will try some more experiments in future weeks. What I would really like to establish is whether heavy boats really go faster in a chop, or whether they just feel faster. GPS could help establish this.

From France: After years designing and building rowing boats I came to this conclusion:Lighter is always faster if you can row well consistantly. Heavier will tolerate a not soo good rowing movement and will give you the feeling of a better ride. Soo the rower is the most important factor, the boat comes second... hard to conceed no?

The other contributors make good points. Don't forget the Pilot Gig has an extreme waterline length to WL beam ratio, has an extremely fine entry and cuts through the initial part of the wave until the bow lifts. They are extemely sea-worthy which bears out Ralph Birds assertion, and I would never have argued with him. The point about difficulty in turning with the ballast on board would result if you put the ballast in the ends of the boat, rather than amidships, under your rowing seat, for instance. With the weight in the ends you not only get the dumb-bell effect, ie a high polar moment of inertia, but it makes the ends less buoyant, causing them to lift more slowly.